1. Field of the Invention
The present invention relates to a sub-mount for an optical receiver using optical communication and an optical receiver using the sub-mount and more particularly to an optical receiver in wavelength selectivity.
2. Description of The Related Art
With the development of communication technology using optical fibers, optical communication is now increasingly employed in subscribers' systems, to say nothing of trunk line systems.
In order to develop such optical communication systems further, optical transmitters as well as optical receivers are needed to be not only smaller in size but also lower in cost. With respect to the optical receiver, there has been examined a surface-mounting type optical receiver as shown in FIG. 4. FIG. 4 is a sectional view along an optical axis.
In German Patent No. DE3543558C2, for example, a V-groove 2 is formed in an silicon substrate 1 so as to fix an optical fiber 3. The V-groove 2 is formed by chemical etching. Receiving light 7 guided through the optical fiber is then radiated from an edge face of the optical fiber into space.
Further, light is reflected obliquely upward from a light reflective surface 4, and then the light is absorbed by the light receiving portion 6 of a light receiving element 5 (due to the p-n junction) and converted into an electric signal. The light reflective surface 4 is formed simultaneously with the V-groove.
Although the electric signal is not shown, it is taken out of electrodes each provided on an rear-surface of the semiconductor light receiving element and the surface of the light receiving portion via an Au wire.
Many Si benches can be formed less costly by lithography from a large silicon wafer. High sensitivity is stably obtainable as an optical fiber. The light receiving element are accurately positioned by forming a packaging position mark of the semiconductor light receiving element simultaneously with the V-groove in each Si bench. Therefore, the arrangement above is outstanding.
Such optical transmitters and receivers are usable in an optical communication system with wavelengths ranging from 1000 nm to 1700 nm. For example, a semiconductor laser using InP or InGaAsP as material is employed on the transmitter side. On the other hand, a semiconductor light receiving element (hereinafter called a photodiode) with mainly Ge, InGaAs or InGaAsP for use as a light receiving layer is employed on the receiver side.
There are also an optical transmission and reception system using two optical fibers: one for transmission and the other for reception, and an optical transmission and reception system using one optical fiber with different wavelengths: one wavelength for transmission and the other for reception.
FIG. 5 shows the latter case wherein wavelength dividers 103 and 105 are used to separate transmission light from reception light. For example, a transmission signal in a 1300 nm optical transmitter 101 is transmitted through the wavelength divider 103, and then the transmission signal propagated in the direction of travel 108 through an optical fiber 104. The propagated transmission signal is reflected from the wavelength divider 105, which has reverse wavelength characteristics. Then, the reflected transmission signal is received by a 1300 nm receiver 107.
On the other hand, a transmission signal in a 1500 nm optical transmitter 106 is transmitted through the wavelength divider 105, and then the transmission signal is propagated in the direction of travel 109 through the optical fiber 104. The propagated transmission signal is reflected from the wavelength divider 103. Then, the reflected transmission signal is received by a 1500 nm receiver 102.
In this case, an entrance of light from its own light source into the receiver portion due to scattering and reflection causes an optical crosstalk, thus resulting in extremely deteriorating reception sensitivity. Therefore, in a case of a transceiver for 1300 nm transmission and 1500 nm reception, for example, a light receiving portion is required not to show sensitivity to 1300 nm light as much as possible.
Moreover, so-called wavelength multiplex communication for use in simultaneously sending a number of optical signals having wavelengths close to each other through one optical fiber is being extensively carried out now.
FIG. 6 shows an example of the wavelength multiplex communication above wherein one optical fiber 104 is usable for transmitting signals having wavelengths ranging from λ1 up to λn over a long section. On a reception side, n of wavelengths are selected by a multi-wavelength divider 111 and received by a plurality of optical receivers.
For example, one wavelength λx is received by the xth optical receiver from above in FIG. 6. Even in this case, in order to provide sensitivity to the wavelength λx received, that is, sensitivity enough to deal with an extremely weak light received, it is needed to lower the sensitivity as much as possible to light having any wavelength other than λx that has not completely been removed by the wavelength divider.
Although FIG. 7 is quite similar to FIG. 6, in place of the multi-wavelength divider, an optical divider 112 for dividing light having every kind of wavelength into 1/n is employed, the optical divider 112 having no wavelength selective function. At this time, only light having a wavelength of λx is allowed to be incident on the xth optical receiver from above in FIG. 7 via a connector with a wavelength selective filter capable of selecting a desired wavelength out of a plurality of signals having wavelengths ranging from λ1 up to λn. Thus, it is needed to give the receiver a function for selecting only one wavelength λx from many wavelengths.
As set forth above, at all cases, a receiver is required to be highly sensitive to only one wavelength but least sensitive to any other wavelength so as to materialize optical communication using a plurality of wavelengths.
However, photodiodes are generally and broadly sensitive to light having wavelengths longer than a wavelength λg corresponding to band gap energy Eg characteristic of the material used. As λg=1670 in the case of InGaAs, for example, the material has high sensitivity to wavelengths ranging from 1000 nm up to 1650 nm.
Consequently, In case where light having a wavelength of 1300 nm is transmitted and light having a wavelength of 1550 nm is received, when the transmitted light having a wavelength of 1300 nm is scattered, reflected, and then returned from the wavelength divider or the optical connecter on the way, the photodiode is sensitive to this light. Therefore, the signal is not accurately reproduced because of a crosstalk.
When four multiplex signals having a plurality of wavelengths including, for example, 1480 nm, 1500 nm, 1520 nm and 1540 nm are transmitted, the photodiode need not be sensitive to any wavelength caused by scattering light generated in the wavelength divider other than the desired one. Notwithstanding, InGaAs is sensitive to all wavelengths.
Similarly, in the arrangement of FIG. 7, since wavelengths are not selected by the optical divider, the InGaAs becomes sensitive to all wavelengths though the satisfactory wavelength selective function is required. In any other light receiving element such as Ge or InGaAsP using as lightly different wavelength band, the element requires a special wavelength selective function likewise.